Gas sensitive field-effect-transistor

ABSTRACT

A gas sensitive field effect transistor comprises a semiconductor substrate that includes a capacitance well, and source and drain regions of a field effect transistor. A gate of the field effect transistor is separated from the semiconductor substrate by an insulator, and a gas sensitive layer separated from the gate by an air gap. The field effect transistor provides an output signal indicative of the presence of a target gas within the air gap to an amplifier, which provides an amplified output signal that is electrically coupled to the capacitance well.

PRIORITY INFORMATION

This patent application claims priority from German patent application10 2005 014 777.1 filed Apr. 1, 2005, which is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

The present invention relates to the field of semiconductor sensors, andin particular to a semiconductive sensor with a gas sensitive fieldeffect transistor.

Gas sensors that utilize the change in work function of sensitivematerials as the physical parameter have a fundamental developmentpotential. The reasons for this relate to their advantages which arereflected in their low operating power, inexpensive fabrication anddesign technology, as well as the wide range of target gases. The lattercan be detected with this platform technology since numerous differentdetection substances can be integrated in these designs.

FIG. 5 illustrates the sensor design based on the principle of aso-called suspended-gate field-effect transistor (SGFET). The bottom ofthe gate electrode, which is raised in this design, has the sensitivelayer on which an electrical potential is generated in response to thepresence of the gas to be detected based on absorption, which potentialcorresponds to a change in the work function of the sensitive material.As a rule, the signals have a level between 50 and 100 mV. Thispotential acts on the channel of the FET structure, thereby changing thecurrent between the source and the drain (I_(DS)). The coupling of thispotential to the source-drain current is as a function of the gatecapacitance C and of the ratios of channel width to channel length W/L:

$\begin{matrix}{I_{DS} \propto {\frac{W}{L} \cdot {c\left( {c\text{:}\mspace{14mu}{area}\text{-}{specific}\mspace{14mu}{gate}\mspace{14mu}{capacitance}\mspace{14mu}{in}\mspace{14mu} F\text{/}m^{2}} \right)}}} & \lbrack 1\rbrack\end{matrix}$

The changed source-drain current is read out directly. Alternatively,the change in the source-drain current is reset by applying anadditional voltage to the raised gate or to the transistor well. Theadditional voltage required represents the readout signal which isdirectly correlated with the work function change in the sensitivelayer.

The SGFET has limitations in regard to the attainable signal quality dueto the geometry required by the dimensioning of the FET structure. Theavailable surface for coupling is limited by theprocess-technology-determined parameter W/L. Also related to the abovefactors, the signal quality is significantly affected by theintroduction of the air gap between the gate and channel regions and theconcomitant reduction of the gate capacitance. The height of the air gapmust allow for sufficiently rapid diffusion of the gas and is in therange of a few p.m.

Use of the CCFET (capacitively controlled FET) design shown in FIG. 6largely eliminates the limitations associated with the SGFET byproviding a more flexible dimensionability. As a result, significantoptimization is enabled in regard to signal quality. In the CCFET, thereadout transistor 5, shown as source S and drain D, is controlled by anoncontacted gate (floating gate). Together with the opposing gateelectrode which has the gas-sensitive layer, the noncontacted gate formsa capacitor arrangement. The surface of the capacitor arrangement isindependent of the readout transistor and can thus be enlarged, therebyproducing improved signal coupling.

However, the fact that parasitic capacitances are present between thefloating gate 2 and the substrate or capacitance well 3 does have adisadvantageous effect even with the CCFET.

If the direct capacitances present in a gas-sensitive field-effecttransistor are shown graphically in equivalent circuits, the diagrams ofFIGS. 7 and 8 can be drawn up for the SGFET and CCFET variants. In FIG.7, the SGFET clearly has a structure which is composed of a seriesconnection of individual capacitances of air gap C_(L) and those of thereadout transistor C_(G).C _(SGFET) =C _(L) ·C _(G)/(C _(L) +C _(G))

For a given sensitive layer and a given transistor, the air gap heightis thus the single variable. In this case, no improvement of the signalcoupling is possible by using an appropriate electrical control. A CCFETstructure, for which a capacitive functional diagram is illustrated inFIG. 6, contains an additional electrode, the so-called capacitance well3 located below the floating gate 2. As a result, the potential at thefloating gate is determined by an expanded capacitive voltage dividerwhich is formed from the air gap capacitance C_(L), the capacitance ofthe gate C_(G), and the capacitance occurring between the floating gateand that due to the capacitance well, as illustrated in FIG. 8. Byenlarging the area forming capacitance C_(L), it is possible to reducethe effect of the parasitic gate capacitance while maintaining the airgap height. The gate capacitance of the readout transistor is—especiallygiven appropriate dimensioning—negligible with good approximationrelative to other capacitances. The capacitance well is used to shieldthe floating gate electrode and is accordingly connected to ground.Assuming the above preconditions, the potential at the floating gateU_(UF) is:ΔU_(FG)=ΔΦ_(S) ·C _(L)·(C _(L) +C _(W) +C _(G))/(C _(G) +C _(W))  [2]The changes in the U_(FG) are converted through the transistorcharacteristic directly into changes in the source-drain current I_(DS)and in response to a given ΔΦ_(s) are a direct measure of the signalobtained with the gas sensor. Based on the introduction of an air gapwith a height of a few μm, the result is:C _(L) <<C _(G) +C _(W)  [3]

The result up to this point is a significant loss of signal.

The two variants according to the equivalent circuits of FIGS. 7 and 8function by coupling the gas signal to the source-drain current I_(DS),used as an example of the measured quantity. The signal is degraded,however, due to the capacitances present. With the SGFET, this factorcan be counteracted by increasing the W/L, and additionally in the CCFETby increasing the surface of the readout capacitance, such that in thelimiting case a very large area is obtained whereby C_(G)<<C_(W).ΔU _(FG)∝ΔΦ_(S) ·C _(L) /C _(W)  [4]

Using parameters possible in a standard CMOS process, in the CCFET aweakening of the signal on the order of 1:10 to 1:100 caused by thecapacitive voltage divider must be expected with the operating methoddescribed above.

There is a need for an improved gas-sensitive field-effect transistorwhich largely eliminates interference effects.

SUMMARY OF THE INVENTION

The invention is based on the knowledge that the circuitry of a CCFET inwhich a reference electrode/capacitance well is provided must beimplemented, when reading out the work function change in gas-sensitivelayers, such that by using appropriate switching measures, such astracking the potentials of certain electrodes, parasitic effects ofcapacitances contained in the design can be reduced. This has a directeffect on the coupling of the signal of the gas-sensitive layer to thecurrent I_(DRAIN/SOURCE), and thus effects an amplification of thesignal of the gas sensor by one to two orders of magnitude.

The effect of adapting or tracking the noncontacting floating gateelectrode and the reference electrode to the same potential is thatparasitic capacitances, such as, for example, capacitance C_(W) betweenthe floating gate electrode and the reference electrode no longer haveany effect, so that ideally the following situation results based onequation [2]:

$\begin{matrix}{{\Delta\; U_{FG}} = {{\Delta\Phi}_{S} \cdot {C_{L}/{\left( C_{G} \right)\mspace{14mu}\lbrack 5\rbrack}}}} & {\operatorname{>>}\mspace{11mu}{{\Delta\Phi}_{S} \cdot {C_{L}/\left( {C_{G} + C_{W}} \right)}}} & {{{in}\mspace{14mu}{analogy}\mspace{14mu}{{to}\mspace{11mu}\lbrack 2\rbrack}}\mspace{14mu}} \\\; & {\operatorname{>>}\mspace{11mu}{{\Delta\Phi}_{S} \cdot {C_{L}/\left( C_{W} \right)}}} & {{in}\mspace{14mu}{analogy}\mspace{14mu}{{to}\mspace{11mu}\lbrack 4\rbrack}}\end{matrix}$

These and other objects, features and advantages of the presentinvention will become more apparent in light of the following detaileddescription of preferred embodiments thereof, as illustrated in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an embodiment of a CCFET with capacitive coupling tofloating gate 2;

FIG. 2 is a schematic diagram illustrating the tracking of referenceelectrode/capacitance well 3 when used, capacitively decoupled,amplified, and for tracking the reference electrode;

FIG. 3 is a schematic diagram illustrating tracking of a referenceelectrode, wherein a signal potential-correlated with the floating gatecan be generated through an additional electrode, in particular, in theair gap;

FIG. 4 is a schematic diagram illustrating the tracking of the referenceelectrode through the output signal of the readout transistor;

FIGS. 5 and 6 are schematic diagrams showing the design of agas-sensitive field-effect transistor, a CCFET being illustrated in FIG.6;

FIGS. 7 and 8 are equivalent circuit diagrams for FIG. 5 or 6; and

FIG. 9 shows an embodiment of the invention for decoupling the potentialof the floating gate electrode, in this case implemented by an annularelectrode surrounding the floating gate.

DETAILED DESCRIPTION OF THE INVENTION

In a CCFET structure, for example that of FIG. 6, an appropriatereference electrode, generally called a capacitance well, is containedin the structure. This electrode is located below noncontacting floatinggate electrode 2. As a result, the potential at the floating gate isdetermined by an extended capacitive voltage divider which is formedfrom the air gap capacitance C_(L), the capacitance of the gate or gateelectrode 6, and the capacitance occurring between the floating gateelectrode 2 and the reference electrode 3. The reference electrode 3defines the potential of floating gate electrode 2, which action occursas a result of the difference between the potentials of the gateelectrode and the reference electrode. A change in the potential at thereference electrode/FG results as defined by equation [2].

FIG. 7 illustrates an equivalent circuit diagram of an SGFET and FIG. 8illustrates an equivalent circuit diagram of a CCFET, FIGS. 2, 3 and 4,as well as FIG. 1, illustrate variants of the invention. It is evidentfrom the prior art reproduced in FIGS. 5 through 7 that, according toFIGS. 5 and 7, no additional parasitic effects influenceable byadditional electrodes are produced between the gas-sensitive layer 1 andthe channel of the field-effect transistor. Only the capacitances of airgap C_(L) and gate C_(G) are present. Based on the diagrams for a CCFETin FIGS. 6 and 8, it is evident that additional capacitance is presentin the design due to the reference electrode/capacitance well 3, whichcapacitance may under certain circumstances have parasitic effects.Capacitance C_(w) is formed between the floating gate 2 and thereference electrode 3.

Due to the tracking of the potential of the reference electrode 3, theparasitic effect of capacitance C_(w), and under certain circumstancesthe effect of the substrate as well, which is generally composed ofsilicon, are eliminated. The result is an improvement in the coupling ofthe gas signal to the channel region of the transistor, and thus a gainin the gas sensor signal by several orders of magnitude.

The three possible variants for implementing the method according to theinvention are shown in FIGS. 2, 3, and 4, as well as in FIG. 1.Fundamentally, the potential of the reference electrode/floating gateelectrode 2 is decoupled and utilized in electrically decoupled form inorder to control reference electrode/capacitance well 3. As a result,losses due to parasitic capacitances are compensated, wherein anintermediate gain by an amplifier 4, which in particular has a highinput resistance and thus functions as an isolation amplifier, and acorresponding switching circuit are controlled. The goal is the trackingof the potential of the reference electrode to the potential of thenoncontacting floating gate electrode.

According to equation [5], the tracking of the voltage U_(w) at thereference electrode 3 can be effected whereby the potential U_(FG) atthe floating gate electrode 2 is capacitatively decoupled, then appliedin electrically decoupled form to the reference electrode.

This decoupling of potential U_(FG) can be effected capacitivelydirectly from the floating gate electrode 2. Alternatively, anadditional electrode, for example, one incorporated in the air gap, canserve as the control electrode, wherein the differences betweenpotentials U_(FG) and U_(w) are compensated through the gain of thecircuit. As a result, the dimensioning of the control electrode 11 isnot strongly tied to the design of the floating gate electrode.

FIG. 4 shows another approach to achieving the tracking of the potentialU_(w) within the gas-sensitive field-effect transistor. For thispurpose, the reference electrode 3 is reached through the amplifiedoutput signal of the sensor or of the readout transistor. The potentialof the floating gate electrode 2 is connected directly to the readouttransistor, and the output signal thereof is in turn connected to theamplifier 4. The amplified signal is supplied to the reference electrode3.

The advantages of the invention provide overall high signal qualitiesfrom the gas-sensitive field-effect transistors, wherein interferenceeffects such as signal drift or noise are suppressed due to improvedcoupling of the work function generated at the gas-sensitive layer tothe transistor current employed as the measured quantity. In addition,the increase in the amplitude of the measurement signal brings about asignificant improvement in the signal-to-noise ratio. Due to theimproved sensor signals, the required evaluation electronics can bedesigned in a more cost-effective manner.

FIG. 1 shows one possible variant of an embodiment with capacitivecoupling to the floating gate electrode. The signal of the controlelectrode 11 is applied to the amplifier 4, the output signal of whichin turn can be supplied to the reference electrode 3.

As indicated in FIG. 9, the decoupling of the potential from thefloating gate electrode can be implemented by an annular electrodeentirely surrounding the floating gate. FIG. 9 provides a top view of achip, wherein a field-effect transistor is mounted to effect thereadout, along with a raised gate, and an electrode for capacitivelytapping the potential at the centrally positioned floating gate.

The electronics required for the elimination of the parasitic componentscan be advantageously integrated as an integrated circuit into the Sichip which contains the FET structure.

Although the present invention has been illustrated and described withrespect to several preferred embodiments thereof, various changes,omissions and additions to the form and detail thereof, may be madetherein, without departing from the spirit and scope of the invention.

1. A gas-sensitive field-effect transistor (FET) comprising: asemiconductor substrate; a gate electrode that includes a gas-sensitivelayer; an air gap between the gas-sensitive layer and the semiconductorsubstrate; a noncontacting floating gate electrode that is capacitivelycoupled to the gate electrode; a field-effect transistor that includesthe floating gate electrode, a source and a drain, and provides a sensedsignal indicative of the electrical potential on the floating gateelectrode; an amplifier that receives the sensed signal andthat-provides an amplified sensed signal; and a reference electrode,which together with the floating gate electrode forms a capacitanceC_(W), and that is electrically coupled to the amplified sensed signal.2. The gas-sensitive field-effect transistor (FET) of claim 1, where anelectronic circuit to reduce parasitic components is integrated into thesemiconductor substrate.
 3. The gas-sensitive field-effect transistor(FET) of claim 2, where in order to read out the potential of thefloating gate electrode, a control electrode in coupling connection withthe floating gate electrode is provided, the potential decoupled fromthe floating gate electrode can be supplied to the amplifier, and theamplified sensed signal is applied to the reference electrode in orderto control the latter.
 4. A gas-sensitive field effect transistor,comprising: a semiconductor substrate that includes a capacitance well,and source and drain regions of a field effect transistor; a gate of thefield effect transistor separated from the semiconductor substrate by aninsulator; a gas sensitive layer separated from the gate by an air gap;an amplifier that receives an output signal from the field effecttransistor and provides an amplified output signal that is electricallycoupled to the capacitance well; where the output signal is indicativeof the presence of a target gas within the air gap.
 5. The gas sensitivefield effect transistor of claim 4, where the substrate is a siliconsubstrate.